An anti-corrosion pretreatment coating is often applied to metal substrates, especially if the substrate will be exposed to the elements in use, prior to the application of outer decorative or protective coatings. These pretreatment coatings are designed to minimize corrosion of the metal substrate, if and when the substrate is exposed to moisture and oxygen. One common metal substrate comprises aluminum or aluminum alloys. These substrates find particular use in the automotive industry, aerospace industry and others wherein a light weight strong metal substrate is required. These substrates are typically initially treated with an alkaline cleaner to remove oil and other surface debris prior to application of a corrosion resistant pretreatment layer. Such alkaline cleaners include Parco® Cleaner 1533 available from Henkel Adhesive Technologies. Typical anti-corrosion pretreatments used after the cleaning step include zinc phosphate based treatments such as the Bonderite® systems or the zirconium oxide based TecTalis® systems. Both of these are available from Henkel Adhesive Technologies. Key to application of these pretreatments is adequate cleaning of the substrates with the alkaline cleaners. In a typical automotive process the substrate is initially cleaned with an alkaline cleaner, rinsed with water, treated with a pretreatment coating, rinsed again with water and then coated in order with an electro-coating, a primer, a base paint coat, and a clear coat.
Aluminum and its alloys are particularly susceptible to corrosion of a filiform type. Filiform corrosion appears as small thread-like formations under organic coatings on aluminum substrates. The initiation is usually at a scratch or defect in the coating. The corrosion filament propagates by an anodic undermining reaction. The aluminum surface at the front of the filament is corroded causing the coating to detach from the substrate and get pushed up by the corrosion products that form in the filament. Typically the corrosion occurs at humidity levels of from 40 to 90% and they follow a pre-existing directionality. It has been observed in the past that the formation of filiform corrosion is especially evident in aluminum alloys that have copper levels of 0.5% by weight or higher. The aluminum substrates are also more susceptible if they experience mechanical stress during the production process such as sanding operations. Although attempts have been made to reduce filiform corrosion, they have not met with complete success and there is a need for improved filiform corrosion resistance particularly in zirconium based coatings and in coatings in general on aluminum alloys having copper levels of greater than or equal to 0.5 weight %.
It is desirable to enhance the corrosion resistance provided to aluminum substrates by standard anti-corrosion pretreatments such as zinc phosphate or zirconium based pretreatments. It is desirable to provide this enhancement in a manner that requires minimal change to existing processes and in a cost effective manner. Preferably the cleaner will also enhance corrosion protection of mechanically stressed aluminum or aluminum alloy substrates. Finally, the cleaner preferably will be applicable to a variety of pre-treated aluminum and aluminum alloy substrates.